I've never thought about a Borg v. Dominion scenario. But doesn't it just seem like the Borg have enough ships that they can just simultaneously swoop in and destroy every Dominion cloning facility at one time? Of course that is assuming they knew where they were all located. But, all it takes it assimilating the right Vorta for that info.
Likewise the Founders could create a Vorta, give it wrong information, make it get captured by the Borg, wait for the Borg ships to show up and detonate the closet star.
Though given the apparent resources of the Borg I've always wondered why they always insisted on the one Cube at a time approach for the Federation.